The illicit distribution of copyright material deprives the holder of the copyright the legitimate royalties for this material, and could provide the supplier of this illicitly distributed material with gains that encourages continued illicit distributions. In light of the ease of transfer provided by e.g. the Internet, content material that is intended to be copyright protected, such as artistic renderings or other material having limited distribution rights are susceptible to wide-scale illicit distribution.
In particular, content items such as music or video items are currently attracting a significant amount of unauthorised distribution and copying. This is partly due to the increasing practicality and feasibility of distribution and copying provided by new technologies. For example, the MP3 format for storing and transmitting compressed audio files has made a wide-scale distribution of audio recordings feasible. For instance, a 30 or 40 megabyte digital PCM (Pulse Code Modulation) audio recording of a song can be compressed into a 3 or 4 megabyte MP3 file. Using a typical 56 kbps dial-up connection to the Internet, this MP3 file can be downloaded to a user's computer in a few minutes. This may for example allow a malicious party could provide a direct dial-in service for downloading an MP3 encoded song. Moreover, the introduction of broadband internet connections stimulates the download of even bigger files such as MPEG video. The illicit copy of the MP3 encoded song can be subsequently rendered by software or hardware devices or can be decompressed and stored on a recordable CD for playback on a conventional CD player.
A number of techniques have been proposed for limiting the reproduction of copy-protected content material. The Secure Digital Music Initiative (SDMI) and others advocate the use of “digital watermarks” to prevent unauthorised copying.
Digital watermarks can be used for copy protection according to the scenarios mentioned above. However, the use of digital watermarks is not limited to copy prevention but can also be used for so-called forensic tracking, where watermarks are embedded in e.g. files distributed via an Electronic Content Delivery System, and used to track for instance illegally copied content on the Internet. Watermarks can furthermore be used for monitoring broadcast stations (e.g. commercials); or for authentication purposes etc.
Techniques have been proposed for embedding watermarks directly in a coded bit stream. This technique is frequently referred to as bitstream watermarking. Further description of bitstream watermarking may be found in PCT Patent Application WO 01/49363 A1‘Method and System of Digital Watermarking for Compressed Audio’ or in ‘Audio Watermarking of MPEG-2 AAC Bitsteams’ by Christian Neubauer and Jurgen Herre, 108th AES Convention, Paris, Feb. 2000. Audio Engineering Society, preprint 5101.
Techniques have further been proposed for embedding watermarks directly in uncompressed signals (also referred to as a base band signal), and there are several known techniques for embedding watermarks in a raw uncompressed signal. For example a watermark may be directly embedded in a PCM (Pulse Coded Modulation) signal which may subsequently be encoded.
An example of a watermarking system for embedding a watermark in a base band signal may be found in “A temporal domain audio watermarking technique” by A. N. Lemma, J. Aprea, W. Oomen, and L. van de Kerkhof, IEEE Transactions on signal processing, Vol 51, No 4, Apr. 2003, page 1088-1097, Institute of Electrical and Electronic Engineers.
Naturally the performance and characteristics of watermark detection processes is a major factor in the success of a watermark based system. A method for detecting watermarks embedded in accordance with the above described approach comprises a two stage approach wherein individual watermark symbols are estimated in the first stage, a plurality of estimated watermark symbols are correlated with a known watermark pattern in the second stage and a detection decision is made depending on the degree of correlation. Further details of this watermark detection method may be found in “A Temporal Domain Audio Watermarking Technique” by A. N. Lemma, J. Aprea, W. Oomen, and L. van de Kerkhof, IEEE Transactions on signal processing, Vol 51, No 4, April 2003, page 1088-1097, Institute of Electrical and Electronic Engineers.
However, although such a detector is useful for watermark detection, it is sensitive to noise which may affect the performance. Noise may e.g. comprise distortions introduced by e.g. common signal processing (e.g. audio compression, dynamic amplitude compression etc) or noise introduced in a broadcast chain. Noise may cause the detector to indicate that a signal comprises a watermark although none is present or the detector may fail to detect a watermark embedded in a signal. Accordingly, it would be advantageous if improved performance and in particular improved detection accuracy could be achieved.
Furthermore, in practical implementations it is important that complexity and computational requirements of the watermark detection is minimised. However, improved performance and reliability of detection is typically achieved at the cost of increased processing and complexity.
Hence, an improved system for watermark detection would be advantageous and in particular a system allowing improved detection performance, reduced complexity and/or facilitated implementation.